Resistive elements of interest to this invention include, for example, NTC thermistors. Patent Document 1 discloses, as a composition constituting a main body element for a NTC thermistor for use as a thermistor for temperature compensation or a thermistor for inrush current suppression (power thermistor), an oxide composition containing at least one of manganese, copper, calcium, cobalt, or nickel, with borosilicate glass added thereto.
Such thermistor materials which have a Mn—Co based spinel structure are widely used in conventional thermistors for temperature compensation or thermistors for inrush current suppression.
In general, circuits as shown in FIG. 10 are used for inrush current suppression. FIG. 10 shows, as a block diagram, an electrical device including a power thermistor for inrush current suppression.
Referring to FIG. 10, an electrical device 11 includes a load circuit 13 driven by an alternating-current power supply 12, and the alternating-current power supply 12 is adapted to supply power through a rectifier 14 to the load circuit 13. A power thermistor 16 for inrush current suppression is connected in series with a power supply line 15 for this power supply. In addition, a smoothing capacitor 17 is connected in parallel to the load circuit 13.
Conventionally, an NTC thermistor is often used as the power thermistor 16. The NTC thermistor exhibits, unlike common solid resistors, a high resistance from power-off to immediately after power-on, and undergoes a decrease in resistance by self-heating after the power-on. Therefore, the NTC thermistor has an advantage of being able to reduce the power consumption, as compared with common solid resistors which undergo almost no change in resistance value depending on temperature changes.
To explain the operation of the circuit shown in FIG. 10 more specifically, (1) the inrush current generated by quickly charging the smoothing capacitor 17 in the case of applying power from the alternating-current power supply 12 is suppressed by the initial resistance R25 (resistance value at 25° C.) of the power thermistor 16 composed of the NTC thermistor; (2) after a steady current flows through the load circuit 13, the power thermistor 16 undergoes a decrease in resistance value as a result of self-heating; and (3) the reduced resistance of the power thermistor 16 can reduce the power loss when the steady current flows, as compared with solid resistors, and as a result, the power consumption can be restrained.
Therefore, the increased difference between the standby (power-off) resistance value at room temperature and the resistance value obtained when the steady current flows (B constant increased) with power thermistor 16 achieves a more beneficial inrush current suppression effect, and makes it possible to further restrain the power consumption in the steady state.
The power thermistor is widely used in power supply devices such as AC adapters. In the case of these applications, a single plate with a lead terminal, which is large in volume, is typically used as the power thermistor in order to withstand the high energy of the inrush current. However, the single-plate power thermistor with the lead terminal does not always serve the need to reduce the circuit in thickness and size, and has disadvantages in terms of mounting cost, etc. Therefore, power thermistors as SMDs (surface-mounted components) have been strongly desired.
However, when such an existing spinel thermistor material as described in Patent Document 1 is used for the power thermistor as an SMD-adaptive small-size chip device, there is a problem of element destruction caused by an inrush current encountered, thereby resulting in a failure to function as an inrush current countermeasure element, and no SMD-adaptive small-size power thermistor has been achieved yet. More specifically, this means that it is not possible to withstand the inrush current unless the element is large in volume in the case of existing materials.
There are several conceivable reasons therefor.
One of the reasons is that existing spinel thermistor materials have relatively high resistivity, and achieve only values on the order of 4000 at most for B constant. In general, insulators and semiconductors exhibit NTC (negative temperature coefficient) characteristics of resistance changed with the increase in temperature, and have a tendency to undergo a substantial change in resistance with respect to temperature as the resistivity is increased, and undergo a decrease in temperature dependence because as the resistivity is decreased, insulators are close to metals in response. More specifically, the B constant is increased as the resistivity is increased, whereas the B constant is decreased as the resistivity is decreased. Ideally, materials are suitable which is lower in resistivity and higher in B constant, while it is difficult to achieve a balance therebetween in the case of existing materials.
Therefore, measures have been taken for existing power thermistors, such as the interelectrode distance and the opposed electrode area respectively shortened and increased in order to lower the resistance as a device, while a thermistor material is selected which is dominated by hopping conduction, slightly high in resistivity, and approximately 3000 in B constant. As a result, conventional power thermistors serve as, for example, large disk-shaped devices.
However, when the inrush current flows in, it is absorbed by decreasing the element resistance while gradually converting the energy into heat, and it is not possible to substantially decrease the resistance, because of the B constant on the order of 3000. As a result, when the element is reduced in volume, element destruction is caused by the thermal energy or by the large inrush current.
In order to achieve SMD-adaptive small-size power thermistors, a novel material is required which can, at the very least, satisfy two conditions of relatively low resistivity and high B constant, and it is difficult to achieve a balance.